Depends. If the machines have similar hardware then the configs are the same as well. But even if they are not, then you just make a general stage4 and put it everywhere. Like this you don't get advantage over tthe binary distros, but still you can have as a lean system you want, which is a valuable advantage as well. but still as smitty3268 pointed, with gcc-4.4 you can use march=native, which helps a lot.
Also, the rolling nature of Gentoo, helps to keep every server and client moden for ever, unlike the conventional distros.
But for sure Gentoo isn't a distro which can be used in your machines in dt time unlike OpenSUSE etc.